As presently planned, in the year 2000 Bluetooth, which is a wireless data communication technique, will be set up as a new short-distance wireless communication system for PCs (personal computers) or peripheral devices. Specifications for Bluetooth can be obtained from, for example, http://www.bluetooth.com/.
Since it is easy to distribute Bluetooth, Bluetooth is considered suitable for use as the physical layer for an ad hoc network. An ad hoc network is one that is freely constructed without a fixed facility being required.
The primary reason for using Bluetooth is to provide a replacement for cables between PCs and peripheral devices. Providing connections between specific devices is also regarded as important. Therefore, the structure of Bluetooth is not always appropriate for the construction of an ad hoc network.
A component of Bluetooth, as is shown in FIG. 1, is a small communication unit called a piconet 100, which comprises a single master radio station (node), 110, and up to seven slave radio stations (nodes), 120, 130, 140, 150, 160 and 170.
Each piconet 100 uses its own hopping sequence to perform frequency hopping, and can not directly communicate with other such communication units.
Therefore, for communication among nine or more radio stations to interconnect piconets, a multi-hop communication form which uses a bridge must be employed, regardless of physical distances involved.
Frequency hopping is a frequency diffusion communication system whereby an oscillating frequency is frequently changed by using a pseudo-random number. The hopping sequence itself is a pseudo-random number that defines the order in which the frequency is changed.
A bridge (or a gateway) is a node that is shared by a plurality of different piconets and that can communicate with all the piconets among which it is shared.
For multi-hop communication, when a sender can not directly communicate with a specific recipient, to enable communication an intermediate node is used to relay packets. When Bluetooth is employed to constitute an ad hoc network encompassing an unspecified number of radio stations, a cluster configuration 200, such as is shown in FIG. 2, is common because of the form of the above described physical layer.
In a cluster network 200, each cluster, 210, 220, 230, 240 or 250, comprises several radio stations (nodes) in a communication area, with one radio station (node), 212, 222, 232, 242 or 252, that is called a cluster head (or a master) at the center.
While the same channel is employed for communication within an individual cluster, a different channel is employed for communication among clusters.
A channel is a logical communication path that is employed by using the same communication procedures (including the hopping sequence) to enable bi-directional communication.
When there is a radio station (e.g., 262 or 264) that can receive signals from two adjacent clusters, this radio station (node) serves as a communication bridge, and reciprocates along the channel between nodes.
The cluster network 200 is constituted by the set of clusters that are connected by the bridges 262, 264, 266, 268, 270 and 272.
According to Bluetooth, a piconet corresponds to a cluster, and a master piconet corresponds to a cluster head. Generally, information concerning nodes that are present in a communication area is not provided for an ad hoc network in advance.
That is, when an ad hoc network is constructed, information concerning nodes that are present in a communication area must be obtained, because such information is not made available to the network in advance.
Further, as time elapses the conditions in a surrounding communication area may be changed by the movement of nodes or clusters; new nodes or other clusters may approach and enter the communication area, or nodes or clusters that are currently communicating members may move outside the area.
In order to maintain or manage a network under such conditions, periodical searches for and monitoring of neighboring nodes is required, and a part of the consumable resources (communication time, electric power, etc.) that are available for processing normal communications must be allocated for these search and monitoring activities.
Therefore, when the frequency at which neighbor node monitoring or search activities are performed is high, the overhead for these operations is increased and can contribute to a substantial reduction in the data rate (the data transfer speed). But when the frequency at which neighbor node monitoring or search activities is low, the locating of new nodes or clusters is delayed, and dynamic network changes can not be promptly coped with.
Under normal network employment conditions, during which node movements do not occur very frequently, in many cases the performance of monitoring or search activities constitutes a non-productive, futile expenditure of time and resources.
Since with Bluetooth at least 10 seconds is required to conduct a single search for neighboring nodes, the above trend is especially marked.
To construct a new network, in consonance with the scale, one to several minutes are required for a search for neighbor nodes, and even after the network is constructed, an extremely large overhead is incurred.
Therefore, if there is no network connection format or a mechanism that provides for the performance of searches and monitoring only when the probability of a topological change is high, and that reduces the frequency at which monitoring or search activities are normally performed, it is difficult to operate an ad hoc network using Bluetooth.
For a network that employs a system other than Bluetooth, the overhead incurred by performing searches for neighboring nodes can also be greatly reduced, and topological changes can be quickly coped with.
Wireless ad hoc communication has been studied since the 1970's as a communication means for use in locations whereat no backbone infrastructure are present, or as an application for portable devices.
An example of a typical ad hoc wireless communication study is reference [1], “Protocols For Adaptive Wireless And Mobile Networking,” D. B. Johnson and D. A. Maltz, IEEE Personal Communications, Vol 3, No. 1, 1996, pp. 34-42. Ad hoc wireless communication has also been continuously studied by the IETF (Internet Engineering Task Force), the association that is responsible for the design and development of the protocol and the architecture for the Internet (e.g., reference [2], “The Zone Routing Protocol (ZRP) For Ad Hoc Networks,” Z. J. Haas and M. R. Pearlman, draft-ietf-manet-zone-zrp-02.txt, 1999, and reference [31], “Ad Hoc On-Demand Distance Vector (AODV) Routing,” C. E. Perkins, E. M. Royer and S. R. Das, draft-ietf-manet-aodv-03.txt, 1999).
For communication for which the cluster structure is employed, see reference [4], “Multicluster, Mobile, Multimedia Radio Network,” M. Gerla and J. T. *C. Tsai, J. Wireless Networks, Vol. 1, No. 3, 1995, pp. 255 to 265, and reference [5], “Cluster Based Routing Protocol (CBRP) Functional Specification, ” M. Jiang, J. Li and Y. C. Tay, draft-ietf-manet-cbrp-spec-00.txt, 1998.
In the above references, [1]to [5], it is assumed that a nearby radio station can be discovered comparatively easily, and substantially, no method for reducing the overhead involved in performing a search is taken into consideration.
For a proximity type wireless system such as Bluetooth, the use of which is expected to spread after the year 2000, a problem arises in that very much labor is required to locate a peripheral radio station because ad hoc communication is not regarded as important (reference [6], Bluetooth Specification Version 1.0A, 1999).
It is, therefore, one object of the present invention to reduce the time required to perform a search for a connectable radio station, so that new wireless ad hoc communication networks can be constructed or so that established networks can be re-constructed in consonance with the movement of a radio station.
It is another object of the present invention to provide a method whereby cluster-based wireless ad hoc communication networks can monitor the quality of received signals and can distribute the signals so as to efficiently search for peripheral radio stations.